1. Field of the Invention
The present invention relates to imaging apparatus with improved resolution, and in particular to a compact imaging apparatus useful in, for example, a mobile telephone or mobile information-processing device.
2. Description of the Related Art
Recent years have seen the appearance of imaging apparatus that captures an image of a subject in the form of digital information. Digital cameras are a notable example, but there is also much interest in incorporating smaller imaging apparatus of this type into portable terminal equipment such as mobile telephones and personal digital assistants (PDAs). As the size of the imaging apparatus decreases, however, picture quality deteriorates because the number of picture elements (pixels) is reduced.
FIG. 1 schematically shows an imaging apparatus comprising a lens system 1, and an imaging device 2 mounted on a supporting substrate 3. The X, Y, and Z axes in the drawing indicate absolute directions: the Z axis indicates the direction of the optic axis; the Y axis indicates the vertical direction as defined by the force of gravity, that is, the direction in which a plumb line would hang. The imaging device 2 comprises an array of pixels aligned horizontally and vertically in the XY plane. The image formed on the imaging device is inverted by the lens system 1 so that when a standing human subject is imaged, for example, the Y-axis arrow in the drawing points from the subject's head toward the subject's feet.
FIG. 2 shows an enlarged schematic view of part of the imaging device 2. The X and Y axes in FIG. 2 match the X and Y axes in FIG. 1. The imaging device 2 comprises an array of photoelectric transducers 2a, each of which converts incident light to an electric signal. The letters R, G, and B in FIG. 2 indicate photoelectric transducers 2a with red filters, green filters, and blue filters, respectively. Each photoelectric transducer 2a constitutes one pixel. The pixels are arranged in a square matrix; that is, they are arranged in straight rows and columns, with equal spacing in the row and column directions.
It is common for each pixel in a color imaging device 2 to have a red, green, or blue filter as shown in FIG. 2. If the purpose of the imaging device 2 is to obtain an image for display on a computer monitor screen, or on any type of liquid crystal display (LCD) screen, it is also common for the pixels to be arranged in a square matrix, because the pixels of the display device have a square matrix arrangement. The pixel signals of the imaging device 2 are read out sequentially in a scanning sequence that advances from the bottom scanning line to the top scanning line, as indicated by the lines C and D with arrows in FIG. 2. The scanning sequence proceeds parallel to the X axis, as indicated by line C, until all pixels in one line have been scanned, then proceeds to the line immediately above, as indicated by line D.
Referring to FIG. 3, the imaging device 2 in FIG. 2 may be structured as a charge-coupled device, also referred to as a CCD image sensor or CCD imager. Reference numeral 2e in FIG. 3 denotes a photodiode that performs photoelectric conversion. Electrical charges that accumulate in the photodiodes 2e are transferred sequentially in the vertical direction through vertical transfer registers 2f, then in the horizontal direction through a horizontal transfer register 2g, and are read out through a readout circuit 2h. The vertical transfer registers 2f are also referred to as interline transfer registers.
FIG. 4 is a block diagram showing how signals are processed in a conventional imaging apparatus. Reference numeral 1 denotes the lens system, 2 denotes the imaging device, 4 denotes a correlated double sampling (CDS) circuit, 5 denotes an amplifier, 6 denotes a signal processing circuit that carries out processes necessary for the amplified signal to be converted into a picture signal, 6a denotes a selector, 6b denotes a synchronizing circuit, and 7 denotes a timing generator (TG).
The correlated double sampling circuit 4 receives a sequence of pixel signals, read out from the imaging device 2 according to driving pulses supplied from the timing generator 7, and eliminates noise components from the pixel signals, leaving only the desired signal component of the subject being imaged. The ‘cleaned-up’ signal output from the correlated double sampling circuit 4 is supplied through the amplifier 5 to the signal processing circuit 6, which includes the selector 6a and synchronizing circuit 6b. From the pixel signals output by the imaging device 2, the selector 6a selects the pixel signals belonging to the effective picture area, that is, the area that will actually be displayed on a display screen (not shown in the drawing).
FIG. 5 shows the relationship between the effective picture area and the effective pixel area in the imaging device 2. Reference numeral 1a denotes the image circle of the lens system 1; reference numeral 2c denotes the effective pixel area of the imaging device 2, that is, the area in which non-masked pixels are disposed; reference numeral 2d denotes the effective picture area. Pixel signals are generated by light incident on any pixels in the effective pixel area 2c, but a valid picture signal is obtainable only within the area covered by the image circle 1a. The effective picture area 2d is therefore smaller than the effective pixel area 2c, being confined to a rectangular area within the image circle 1a. From the signals output by pixels in the effective pixel area 2c of the imaging device 2, the selector 6a selects the signals output by pixels disposed in the effective picture area 2d, and supplies them to the synchronizing circuit 6b. 
The signal generated by a pixel includes information for only one color: red, green, or blue. To obtain complete red, blue, and green signals, the synchronizing circuit 6b interpolates the values of the color signals missing from each pixel. For example, the synchronizing circuit 6b provides green and blue signal values for pixels having red filters.
One method of improving the picture quality of a square pixel matrix of the type described above is to increase the resolution by increasing the pixel density, thereby increasing the number of pixels in the matrix. If the pixel density is increased, however, each pixel has less light-receiving area, causing an inevitable loss of sensitivity. Thus the approach to improved picture quality through higher pixel density entails an inherent compromise between sensitivity and resolution.
To circumvent this compromise, another method of obtaining higher resolution and better picture quality has been proposed. FIG. 6 is a graph shown in “High-Definition Still Image Processing System Using a New Structure CCD Sensor”, Proceedings of SPIE, Vol. 3965 (2000), pp. 431–436, which illustrates the frequency characteristics of subjects imaged in natural scenes. FIGS. 7 to 10 are graphs illustrating frequency characteristics of a square matrix of imaging elements of the type shown in FIG. 2. FIG. 7 shows the limit spatial frequency range of a black-and-white picture imaged by imaging elements arranged in a square matrix, FIG. 8 shows the limit spatial frequency range of the green signal, FIG. 9 shows the limit spatial frequency range of the red signal, and FIG. 10 shows the limit spatial frequency range of the blue signal. A comparison of FIG. 7 with FIG. 6 shows that while natural subjects tend to have the widest spatial frequency ranges in the horizontal and vertical directions, the widest frequency ranges of a square pixel matrix are oriented at angles of forty-five degrees to the horizontal and vertical axes. There is thus a major mismatch between the frequency characteristics of the imaging device and the frequency characteristics of the subjects typically imaged. This suggests that the frequency characteristics of the image in the horizontal and vertical directions can be improved by changing the pixel arrangement.
For example, FIG. 11 shows a honeycomb arrangement of imaging elements disclosed in Japanese Unexamined Patent Application Publication No. 11-168688. In this arrangement the pixels are offset horizontally and vertically, the horizontal spacing between pixels being increased from PH to √2·PH, and the vertical spacing between pixel rows being decreased from PV to PV/√2. This honeycomb arrangement is obtainable from the square matrix arrangement shown in FIG. 12 by tilting the matrix at an angle of forty-five degrees, as shown in FIG. 13. If the pixel column spacing PH and pixel row spacing PV in FIG. 12 are both equal to N, then the pixel column spacing and pixel row spacing in FIG. 13 are both reduced to N/√2. The offset arrangement therefore has higher resolution in the horizontal and vertical directions.
FIGS. 14 to 17 are graphs illustrating frequency characteristics of the offset arrangement. FIG. 14 shows the limit spatial frequency range of a black-and-white picture imaged by imaging elements arranged in the offset arrangement, FIG. 15 shows the limit spatial frequency range of the green signal, FIG. 16 shows the limit spatial frequency range of the red signal, and FIG. 17 shows the limit spatial frequency range of the blue signal. Whereas the maximum horizontal and vertical spatial frequencies resolvable by the square matrix were 1/N (FIG. 7), the maximum horizontal and vertical spatial frequencies resolvable by the offset arrangement are increased to √2/N (FIG. 7). The offset arrangement therefore provides improved frequency characteristics in both the horizontal and vertical directions. FIG. 18 shows the spatial frequency characteristic of the square matrix and the spatial frequency characteristic of the offset arrangement superimposed on one another. The frequency characteristic of the offset arrangement (hatched) can be seen to resemble the frequency characteristics of natural subjects more closely than does the frequency characteristic of the square matrix.
The offset arrangement of imaging elements shown in FIG. 11 can therefore provide improved resolution in typical pictures of natural subjects. The problem is how to read out the pixel signals. In the Japanese Unexamined Patent Application cited above, the vertical transfer registers 2f zigzag around the photodiodes 2e as indicated by the arrows in FIG. 11. To reach the horizontal transfer register 2g, the electric charges must therefore travel further than in a square matrix, in which the vertical transfer registers 2f are aligned in straight columns, and more vertical transfer registers 2f are necessary than in a square matrix. The extra distance impairs the efficiency of the charge transfer process, and the increased number of vertical transfer registers 2f makes the structure of the imaging device more complex, hence more difficult to manufacture, than a conventional square matrix.